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Name(s): Period: Date: Karyotypes: Diagnosing Chromosome Disorders HASPI Medical Biology Lab 17 Background/Introduction DNA, Genes, and Chromosomes All cells contain genetic information in the form of deoxyribonucleic acid (DNA) molecules. DNA contains the genetic instructions that form a species’ characteristics. DNA is made up of four nucleotides: adenine, thymine, guanine, and cytosine. The arrangement, or order, of these nucleotides creates the directions for a specific protein. Genes are regions in the DNA that contain the instructions that code for the formation of proteins. http://www.futurehumanevolution.com/wp-content/uploads/chromosomesgenes-nucleotides-dna-base-pairs-and-the-future-of-human-evolution1.png Humans have more than 20,000 genes. All cells in an organism have the same genetic content, but the genes used (expressed) by the cell may be regulated in different ways. For example, the gene to create the actin and myosin proteins that are capable of contracting are expressed in muscle cells, but are dormant (not expressed) in skin cells. Not all DNA codes for a protein. Some segments of DNA are involved in regulatory or structural functions, and some have no as-yet known function. In each cell, DNA, and therefore genes, are wound into tight structures called chromosomes. Chromosomes are primarily formed during mitosis, or cell division, to prevent damage or loss of delicate DNA strands. Each chromosome consists of a single, very long DNA molecule. Humans have 23 pairs of chromosomes, 22 pairs of autosomes, and 1 pair of sex chromosomes. As their name implies, sex chromosomes contain the genes that primarily impact male and female characteristics. Gene Expression and Regulation How does a cell know when to express a gene? Some genes are always “on,” and are responsible for cell processes such as metabolism, repair, and protein synthesis. Other genes are regulated and have the ability to be turned “on” and “off.” For example, the gene that codes for insulin is only turned “on,” or expressed, by pancreatic cells when it receives a signal from the brain. The brain only sends the signal when it senses a high level of glucose in the blood. As soon as the glucose level in the blood drops, the brain sends a signal to the pancreatic cells to stop using the gene for insulin, and the gene is turned “off.” Karyotypes A genetic mutation that occurs when chromosomes do not separate correctly during cell division can be observed by looking at human DNA when it is in chromosome form. Observation can determine if there are any extra chromosomes, or missing or damaged chromosomes. This is done by removing the chromosomes from a cell, spreading them out, and making them visible since they are microscopic. A picture is then taken and a http://www.biotechnologyonline.gov.au/images/contentpages/karyotype.jpg Karyotypes: Diagnosing Chromosome Disorders, HASPI Medical Biology Lab 17 445 Name(s): Period: Date: genetic counselor orders the chromosomes into the 22 pairs of autosomes and 2 sex chromosomes. This ordering and pairing of chromosomes is called a karyotype. A normal karyotype is shown on the previous page. The dark or light areas on the chromosomes are areas where the DNA is more or less tightly coiled up. Over 500,000 karyotypes are performed in the U.S. every year, especially for genetic and reproductive medicine. Collecting cells from an unborn fetus, or the amniotic fluid surrounding a fetus, for karyotyping is a common occurrence for expecting parents who wish to make sure no genetic abnormalities exist. The common process for obtaining these cells occurs through amniocentesis, in which a needle is inserted into the womb. There are risks of miscarriage or other damage to the fetus associated with the amniocentesis procedure, so it is usually not chosen to be performed unless there is a suspected possibility of genetic abnormalities. Chromosome Mutations http://www.goldiesroom.org/Multimedia/Bio_Images/19%20Applied %20Genetics/05%20Chromosome%20Mutations. Chromosome mutations can occur when entire chromosomes are duplicated or deleted. They may also occur when sections of chromosomes are duplicated, deleted, inverted, inserted into a different chromosome, or translocated with another chromosome. The image to the left diagrams these occurrences. As you can see, chromosomal mutations change the size and/or organization of the light/dark bands on a chromosome, making it easy to see that a mutation has occurred when completing a karyotype. Review Questions – answer questions on a separate sheet of paper 1. What are the four nucleotides that make up DNA? 2. Within the human body, where is DNA located? 3. What are genes? What do genes contain instructions to create? 4. Do all genes code for proteins? Explain your answer. 5. What are chromosomes? 6. How many chromosomes do humans have? 7. Where did you get your chromosomes? 8. What is the difference between autosomes and sex chromosomes? 9. How is gene expression regulated? 10. What is a karyotype? 11. What is the purpose of performing a karyotype? 12. Hypothesize how the chromosomes of the parents would look compared to the chromosomes of their child. 13. Make a table that describes and depicts (draw a picture) the different types of chromosome mutations. 446 Karyotypes: Diagnosing Chromosome Disorders, HASPI Medical Biology Lab 17